1. Field to which Invention Relates
The present invention relates to an apparatus for stacking and unstacking sheet material, more particularly glass sheets, comprising one or more suction plates, capable of moving towards a sheet material stack the position of the surface of which changes as the sheets are stacked or unstacked, which apparatus is adapted to be moved by a pressure cylinder actuator, and with a sensing member, which on making contact with the stack, generates a pulse adapted to control the piston rod of the pressure cylinder actuator.
2. The Prior Art
Forms of apparatus of this type are used for example for the stacking and unstacking of glass sheets or panes when the glass sheets are placed individually in production premises on a horizontal conveyor and are stacked on a stack standing generally vertically or when the glass sheets are removed individually from the stack.
Since the height of the sheet stack is changed with the stacking of every new sheet, the suction plate or, respectively, the frame or arm carrying the suction plate or plates must travel through a distance which is continuously changing. In the case of previously proposed stacking apparatus the above-mentioned sensing member serves for controlling this distance travel. For this purpose mechanical sensors or feelers have been used, for example in the form of limit switches, which on contact with the sheet surface provide the control pulse. Apparatus of this type is described for example in the German Patent Specifications (Offenlegungsschriften) Nos. 2,130,591 and 2,130,592 and the German Patent Specification (Auslegeschrift) No. 1,155,396.
The limit switches or other mechanical feelers are arranged in the case of the known apparatus in such a manner that they either make contact with the uppermost glass sheet themselves or that they are actuated by the frame carrying the suction plates and in this case the frame is mounted in a yielding manner on the arm carrying the frame so that owing to the relative movement between the suction frame lying against the glass sheet stack and the supporting arm actuation of the end switch becomes possible. In both cases it may be that the mechanical feeler becomes loose or owing to unintentional external action is caused to change its position. The consequence of this will be that the relationship between the position of the feeler and the position of the suction plates is no longer the optimum position just at that instant in which the control pulse for engagement and backward movement of the suction plates is generated so that disfunction of the apparatus may occur. Furthermore, in the case of previously proposed forms of apparatus other causes may lead to the suction plate not making reliable engagement with the sheet to be removed but nevertheless the sensing member generates the control pulse so that for this reason as well disfunction is unavoidable.